Making batteries safer by understanding the important thing thermodynamic variations between the most well-liked cell chemistries
Whereas NMC cells dominate the present era of US and European BEVs, rising price pressures are forcing OEMs to take a tougher have a look at LFPs. Regardless of their decrease vitality density, LFP cells are nonetheless vulnerable to single-cell thermal runaway, and controlling its propagation to adjoining cells stays a prime precedence. To correctly handle thermal runaway propagation (TRP), pack designers should first perceive the three key thermodynamic variations between the 2 cell sorts.
First, NMC cells set off extra simply. The chart beneath reveals two equally sized prismatic cells triggered into thermal runaway with an exterior heating pad. The NMC cell goes unstable at a temperature of 160°C, whereas the LFP holds regular as much as 230°C. This deeper thermal effectively, mixed with LFPs’ better dimension and weight for a similar vary, offers an LFP pack with extra thermal mass than its NMC counterpart. The thermal mass of adjoining cells is without doubt one of the few benign pathways that stray vitality can take throughout a runaway occasion (the opposite being rejection by means of the pack’s cooling system).  Whereas cell-to-cell conduction by itself can not soak up the total brunt of a runaway occasion, thermal mass can present extra time for occupants to tug over and get away from the car.
Second, NMC cells burn hotter than LFPs. As soon as pushed into thermal runaway, the NMC cell-face temperature peaks at 800°C, whereas the LFP solely spikes to 620°C. That is considerably paradoxical since, regardless of their decrease vitality density, calorimetry reveals that LFP cells typically have the next gas content material per amp-hour of storage capability. Nevertheless, as a result of NMCs smuggle extra elemental oxygen into the cell, their combustion effectivity and, subsequently, warmth launch is greater.
Lastly, the ejection phenomena related to every cell chemistry are very completely different. When an NMC cell goes into thermal runaway, there tends to be a 10- to 30-second interval wherein liquid, gasoline, and strong supplies are violently ejected by means of the cell vent. These strong supplies are sometimes bits of aluminum, carbon, and burning plastic. So NMC cells carry all three parts of the hearth triangle – gas, oxygen, and an ignition supply – to a thermal runaway occasion. The ensuing torch-and-grit blast can burn by means of all however the sturdiest enclosure supplies and blankets the encompassing cells in flaming gases and particles. Managing these flows is without doubt one of the most troublesome elements of designing a TRP-resistant NMC pack.
In distinction, LFP cells are likely to emit largely smoke and gasoline which, though sizzling, is usually not actively combusting. Whereas subsequent combustion and even explosions are potential, the inside of an LFP pack is usually oxygen-starved throughout a thermal runaway occasion, so these dangers exist primarily exterior the car. Additional, the full mass ejected from an LFP thermal runaway is barely 20-25% of the unique cell mass, versus 40-50% for an NMC. So, each the hazard stage and amount of LFP ejecta are decrease than that of NMC designs.
Briefly, controlling thermal propagation in an NMC pack is primarily about gasoline administration and secondarily about direct cell-to-cell warmth switch. In an LFP pack, that dynamic is reversed. That is lucky: with the arrival of aerogel thermal obstacles reminiscent of PyroThin cell obstacles, cell-to-cell warmth switch is now a solved drawback. Managing combustion gases and flaming ejecta stays stubbornly arduous to handle.
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